Knee joint prostheses are typically employed in patients suffering serious afflictions of the knee joint, e.g. caused by diseases such as osteoarthritis. Such prostheses typically involve a partial or full replacement of the knee joint, and the components of the prosthesis are fully implanted within the body of the patient. In other words, these knee joint prostheses are generally endoprostheses.
Each year over 100,000 knees are endoprosthetically treated in Germany alone, and of the knee joint prostheses implanted, complications associated with the prostheses arise in a significant proportion of cases and necessitate re-treatment of the patients. The causes of the complications are numerous and include infection, wear and even failure of the prosthesis components. One of the main causes is loosening of the implant, which occurs in about 26% of cases, and studies have shown this often to be directly associated with wear of the implant components.
Conventional knee joint prostheses are typically designed to provide a combination of both hard and soft components. For example, the functional surfaces of the prostheses are typically provided by components respectively fabricated from cobalt-chromium alloys (CoCr) on the one hand, and polyethylene (PE) on the other, thus providing a sliding interface between components of these two materials (CoCr-PE). With numerous patients susceptible to allergic reactions from nickel- and cobalt-chromium based alloys, however, titanium (Ti) is often employed as an alternative, such that the prostheses then have titanium-polyethylene (Ti-PE) material interfaces. A primary reason for the loss of stability in the anchorage of the implant components and their consequent loosening has been indentified to be associated with the accumulation of polyethylene wear particles in the joint. As a result of immunologically induced, inflammation-like reactions in the patient's body, the fragments or particles of polyethylene can activate osteoclasts, which act to remove or degrade bone tissue in the joint. Thus, the wear of the softer polyethylene components has been found to result in a reduced service life of the endoprosthesis, which is rarely more than about ten years.
In view of their excellent wear-resistance properties, ceramic materials would appear to be highly advantageous for use in such endoprostheses. Ceramic materials have the disadvantage, however, that they are relatively brittle (i.e. they have low fracture toughness), making them highly susceptible to fracture when stressed. As a result, ceramic materials are particularly sensitive to the formation of stress concentrations.
The geometry of conventional knee joint prostheses is, not surprisingly, based upon the physiological geometry of the human knee joint, which comprises femoral and tibial condyles. In this connection, the femoral condyles may be generally described in the sagittal plane (i.e. an antero-posterior direction) as having two distinct radii. Thus, the radius of that part of the condyles which is in contact in the knee joint when the leg is in extension is distinctly different to the radius of that part of the condyles which is in contact in the knee joint when the leg is in flexion. The tibial condyles are lightly curved and combine with the meniscus to form a plateau having a somewhat flatter character for receiving the end of the femur. The geometry of the femoral condyles and the associated tibial plateau result in a specific motion of the joint, in which the femoral condyles partly slide upon the tibial plateau, but also roll in a rearward or posterior direction at higher angles of flexion. In order to reproduce the kinematics of the human knee joint in an endoprosthesis, corresponding radii combinations are typically incorporated into the prosthesis designs so that a combined sliding and rolling movement is achieved.
One of the reasons why the use of ceramic materials has not been broadly and successfully implemented in knee joint endoprostheses to date is related to the complex geometry and the rolling contact between the prosthesis components. Such rolling contact creates localized loading of the contacting surfaces of the prosthesis, which in turn creates stress concentrations and can lead to component failure in ceramic materials. In addition, the required level of geometric precision and surface smoothness is often not achieved in the finishing processes for ceramic components, and unevenness in the ceramic contact surfaces can similarly lead to high stress concentrations under load.
Ceramic components are shaped in a “green” format and are subsequently sintered. Due to the loss or reduction in the volume of the ceramic material which inevitably occurs during the sintering process, the dimensional variations or tolerances of the component are relatively high, i.e. between about 2% and 5% under DIN 40680 (where DIN is Deutsche Industrienorm). Furthermore, after sintering, the ceramic is hard and can only be processed with specialised tools, e.g. incorporating diamond abrasives. In this regard, also, it will be noted that highly polished sliding surfaces are required in a knee joint endoprosthesis to ensure that the degree of wear is as small as possible. Consequently, the requisite degree of precision in the dimensioning and finishing of the ceramic components is simply not possible without the ceramic material undergoing a machine finishing procedure.
The kinematics and the relative complex geometries of the human knee joint, in combination with the requirements of implant quality, have impeded the application of ceramic materials in knee joint endoprostheses to date. That is, suitable processing technologies for generating and finishing the complex geometries required for a practically viable knee joint prosthesis formed of ceramic material have not been available. In this regard, basic mechanisms in grinding and polishing of ceramic free-form surfaces are currently the subject of on-going research.
Nevertheless, some attempts to develop knee joint prostheses which employ ceramic materials have been made. For example, in the International Patent Application published as WO-01/30277 A1, a knee joint endoprosthesis having a ball-shaped femoral component and a corresponding spherical socket in a tibial component is described. Such a joint, however, provides the articulation of a fixed hinge, which is biomechanically quite unsuitable for a human knee joint. The International Patent Application published as WO-2006/130350 A2, on the other hand, describes a knee joint endoprosthesis formed from a particular ceramic material having an especially high fracture toughness. This prosthesis, however, still suffers from the problem that the femoral component is designed to provide a rolling motion relative to the tibial component, and this leads to stress concentrations between the engaging surfaces.
The present invention is directed to the development of a new and improved knee joint prosthesis, and in particular to a partial or total knee joint endoprosthesis, which aims to provide increased service life and reduced incidences of component loosening through superior wear properties. At the same time, the invention aims to provide a knee joint prosthesis which is able to meet increasing patient expectations of good joint mobility. The present invention is also directed to a new method of producing components of a knee joint prosthesis which have desired surface characteristics, and to an apparatus for carrying out such a method.